1. Field of the Invention
The present invention relates to a thin film transistor (hereinafter referred to as a “TFT”) and a semiconductor device including a circuit constituted by thin film transistors. The present invention also relates to, as the semiconductor device, an electro-optical device typified by, for example, a liquid crystal panel, and an electronic equipment including such an electro-optical device as a part. Incidentally, the term “semiconductor device” in this specification indicates any devices functioning by using semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic equipment are also semiconductor devices.
2. Description of the Related Art
In recent years, attention has been paid to an active matrix type liquid crystal display device in which its circuit is constituted by TFTs using polysilicon films. This device controls an electric field applied to a liquid crystal in a matrix state by a plurality of pixels arranged in a matrix form, and realizes an image display with high fineness.
In such an active matrix type liquid crystal display device, as the resolution becomes high fineness like XGA or SXGA, the number of pixels alone comes to exceed one million. A driver circuit for driving all the pixels is formed of very complicated and numerous TFTs.
The specification required for an actual liquid crystal display device (also called a liquid crystal panel) is severe, and in order that all pixels normally operate, high reliability must be secured for both the pixels and the driver. Particularly, if an abnormality occurs in the driver circuit, there occurs a bad state called a linear defect in which all pixels of one column (or one row) are destroyed.
However, a TFT using a polysilicon film is generally considered to be inferior in reliability to a MOSFET (transistor formed on a single crystal semiconductor substrate) used for an LSI or the like. Such a view becomes strong that it is difficult to form an LSI circuit by TFTs unless this weak point is overcome.
As a structure for improving the reliability of a TFT, GOLD (Gate Overlapped Light-doped Drain), LATID (Large-Tilt-Angle Implanted Drain), and the like are known. The feature of these structures is that an LDD region and a gate electrode overlap with each other, and by doing so, it becomes possible to reduce an impurity concentration of the LDD region, an effect of relieving an electric field becomes large, and hot carrier resistance becomes high.
For example, in “M. Hatano, H. Akimoto, and T. Sakai, IEDM97 TECHNICAL DIGEST, p523-526, 1997”, a TFT with the GOLD structure is realized by using a side wall formed of silicon.
However, the GOLD structure disclosed in the paper has a problem that an off current (current flowing when a TFT is in an off state) becomes large as compared with a normal LDD structure, and a countermeasure for that has been required.